Mr Riddle was suspended after last year's High Court ruling which found the council had lied to developer Slough Estates over key planning issues. It ordered the council to pay£49 million in damages.
Mr Riddle was not in post at the time of the original decision but was still suspended. His period of suspension ended last week, but he has gone on leave by mutual agreement.
Borough solicitor John Anderson, also suspended, has taken early retirement with the agreement of the council.